Gas sensor

ABSTRACT

A gas sensor has a porous protective layer disposed on a surface of a sensor element. In this gas sensor, the porous protective layer includes ceramic particles and ceramic fibers. Further, the ceramic fibers are present in the porous protective layer over a range from a front surface to a back surface thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of PCT/JP2019/000584,filed on Jan. 10, 2019, the contents of which is hereby incorporated byreference.

TECHNICAL FIELD

The disclosure herein discloses art related to a gas sensor.

BACKGROUND ART

A gas sensor in which a surface of a sensor element is protected by aporous protective layer is known. This type of gas sensor is disposedfor example in an exhaust system of a vehicle having an engine, and isused for measuring a gas concentration of specific gas included inexhaust gas. Japanese Patent Application Publication No. 2014-98590(hereinbelow termed Patent Document 1) describes a gas sensor of whichporous protective layer has a two-layer structure. The gas sensor ofPatent Document 1 is configured such that a porosity of a lower layer(layer in contact with a sensor element) is higher than a porosity of anupper layer (layer exposed to an exhaust gas space), by which it catchesharmful components included in exhaust gas at the fine upper layer(having a lower porosity) and increases heat insulation performance ofthe porous protective layer. Further, in the gas sensor of PatentDocument 1, since strength of the lower layer decreases as the porosityof the lower layer is increased, this decrease in the strength isaddressed by adding ceramic fibers in the lower layer.

SUMMARY OF INVENTION Technical Problem

In the gas sensor of Patent Document 1, the porous protective layerhaving the two-layer structure in which the upper layer is constitutedsolely of ceramic particles and the lower layer is constituted ofceramic particles and the ceramic fibers is used to protect the sensorelement. However, Patent Document 1 is insufficient in regard to itscountermeasure to a case in which the porous protective layer itself isdamaged. That is, the sensor element can be protected when the porousprotective layer is not damaged, however, when the porous protectivelayer is damaged, the harmful components could enter the porousprotective layer from its damaged portion, and as a result the harmfulcomponents could make contact with the sensor element and the sensorelement may thereby be damaged. The disclosure herein aims to provideart for increasing durability of a porous protective layer in a gassensor provided with the porous protective layer.

Solution to Technical Problem

A gas sensor disclosed herein may comprise a porous protective layerdisposed on a surface of a sensor element. In this gas sensor, theporous protective layer may include ceramic particles and ceramicfibers, and in a thickness direction of the porous protective layer, theceramic fibers may be present in a front surface of the porousprotective layer and on a back surface side of the porous protectivelayer relative to a midpoint in the thickness direction. By having theceramic fibers included in the front surface (surface exposed tooutside) of the porous protective layer, this gas sensor can increasestrength of a portion (that is, the front surface of the porousprotective layer) that could be a starting point of damage caused byharmful components (such as metal and moisture) in exhaust gas. Further,in the thickness direction of the porous protective layer, since theceramic fibers are included on a front surface side (that is, in thefront surface) and on the back surface side relative to the midpoint inthe thickness direction, a difference in firing shrinkage amountsbetween the front surface side and the back surface side is reduced, andgeneration of a crack can be prevented.

The “porous protective layer including the ceramic particles” refers notonly to a configuration in which the ceramic particles are present inthe porous protective layer as “particles” but also a configuration inwhich the ceramic particles are present in the porous protective layerin a state of “sintered body” having been sintered. The ceramicparticles are present in the porous protective layer as a matrix (basematerial) of the porous protective layer or as a bonding material thatbonds basic materials constituting the porous protective layer. Further,the “ceramic fibers being present in the front surface of the porousprotective layer” means that the ceramic fibers are present in a layerthat is located frontmost when the porous protective layer is equallydivided into five layers along the thickness direction. That is, it isnot necessarily limited to a configuration in which the ceramic fibersare exposed on the front surface of the porous protective layer (beingin contact with a space outside the porous protective layer).

In the above gas sensor, the porous protective layer may includeplate-shaped ceramic particles. The “plate-shaped ceramic particles”refer to ceramic particles having an aspect ratio of 5 or more and alongitudinal size of 5 μm or more and 50 μm or less. By adding theplate-shaped ceramic particles in the porous protective layer, decreasein strength of the porous protective layer can be mitigated and also apart of the ceramic fibers to be added to the porous protective layercan be omitted. That is, a part of the ceramic fibers in the porousprotective layer can be replaced with the plate-shaped ceramic particleswhile the strength of the porous protective layer is maintained.Typically, a length (longitudinal size) of the plate-shaped ceramicparticles is shorter than a length of the ceramic fibers. Due to this,by replacing a part of the ceramic fibers with the plate-shaped ceramicparticles, a heat transfer path within the porous protective layer canbe obstructed and heat transfer within the porous protective layer tendnot to occur. As a result of this, heat insulation performance for thesensor element can further be increased.

When the plate-shaped ceramic particles are included in the porousprotective layer, in the thickness direction of the porous protectivelayer, the plate-shaped ceramic particles may be present on the backsurface side relative to the midpoint in the thickness direction. Asaforementioned, the plate-shaped ceramic particles contribute toobstruct the heat transfer path in the porous protective layer. Due tothis, by virtue of the presence of the plate-shaped ceramic particlesnear the sensor element (on the back surface side of the porousprotective layer relative to the midpoint in the thickness direction),the heat insulation performance for the sensor element can efficientlybe exhibited.

In the above gas sensor, a ceramic fiber content may be higher on thefront surface side of the porous protective layer than on the backsurface side of the porous protective layer. When a crack is generatedon the front surface side of the porous protective layer, harmfulcomponent enters the porous protective layer from this crack and adamage to the porous protective layer worsens. By having the ceramicfibers in a higher content on the front surface side, which could be astarting point of the damage in the porous protective layer, than on theback surface side, the damage to the porous protective layer canefficiently be reduced without increasing an amount of the ceramicfibers.

In the above gas sensor, the porous protective layer may comprise afirst layer located on the back surface side and a second layer locatedon the front surface side. In this case, a porosity of the first layermay be higher than a porosity of the second layer. That is, the porousprotective layer may comprise a multi-layer structure, and the porositymay be higher in a layer on the back surface side (first layer) than ina layer on the front surface side (second layer). By configuring theporous protective layer to comprise the multi-layer structure,characteristics of the porous protective layer on the front surface sideand on the back surface side (characteristics attributable tocompositions thereof) can easily be controlled. Further, by configuringthe porosity on the back surface side (near the sensor element) to behigh, the heat insulation performance for the sensor element isincreased and a damage to the sensor element is mitigated. A third layermay be disposed between the first and second layers. In this case, thethird layer may have the same characteristic as the first or secondlayer, or may have a different characteristic from the first and secondlayers.

When the porous protective layer comprises the multi-layer structureincluding the first and second layers, the first layer may include theceramic fibers. In this case, a volume fraction of the ceramic fibers inthe first layer may range from 5 vol % to 25 vol % relative to a totalvolume of the ceramic particles, the plate-shaped ceramic particles, andthe ceramic fibers. Heat insulation performance of the first layer maybe prevented from decreasing while strength of the first layer ismaintained.

When the porous protective layer comprises the multi-layer structureincluding the first and second layers, the first layer and the secondlayer may be in contact with each other. That is, the porous protectivelayer may comprise a two-layer structure constituted of the first layeron the back surface side and the second layer on the front surface side.The number of layers for configuring the porous protective layer isminimized and manufacturing process can be prevented from becomingcomplicated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a primary part of a gas sensor;

FIG. 2 schematically shows a porous protective layer;

FIG. 3 shows a composition of the porous protective layer of anembodiment; and

FIG. 4 shows an evaluation result of the embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, an embodiment of a gas sensor disclosed herein will bedescribed. The gas sensor disclosed herein is used for detecting aconcentration of a specific component in air. For example, the gassensor disclosed herein is used as a NO_(x) sensor configured to detecta NO_(x) concentration or an air-fuel ratio sensor (oxygen sensor)configured to detect an oxygen concentration in exhaust gas in a vehiclehaving an engine.

The gas sensor may include a sensor element and a porous protectivelayer configured to protect the sensor element. The sensor element maybe an element configured to detect a concentration of gas to bedetected, and may be in a stick shape. A detection unit configured todetect the gas to be detected may be disposed at one longitudinal end ofthe stick-shaped sensor element. Further, the sensor element may includea heater therein. The porous protective layer may be disposed on asurface of the sensor element. The porous protective layer may bedisposed on a part of the surface of the sensor element, and may bedisposed at least on a surface of the detection unit. The porousprotective layer may cover an entirety of the surface of the detectionunit. Further, a thickness of the porous protective layer may, althoughthis depends on an environment in which it is used, be 100 μm or moreand 1000 μm or less, for example. When the thickness of the porousprotective layer is too thin, its function to protect the sensor elementcannot be sufficiently exhibited, and when it is too thick, thethickness obstructs flow of the gas to be detected into the sensorelement. The porous protective layer may be constituted of ceramics, andmay be constituted of ceramic particles, plate-shaped ceramic particles,and ceramic fibers.

The ceramic particles may be used as a bonding material that bonds basicmaterials constituting basic framework of the porous protective layer,such as the plate-shaped ceramic particles and the ceramic fibers. Metaloxide that is chemically stable in high-temperature exhaust gas may beused as a material of the ceramic particles. Such metal oxide mayinclude alumina (Al₂O₃), spinel (MgAl₂O₄), titania (TiO₂), zirconia(ZrO₂), magnesia (MgO), mullite (Al₆O₁₃Si₂), and cordierite(MgO.Al₂O₃.SiO₂). The ceramic particles may be in a form of particles,and a size thereof (average particle diameter before firing) may be 0.05μm or more and 1.0 μm or less. When the size of the ceramic particles istoo small, sintering in a manufacturing process (firing) of the porousprotective layer may progress excessively, and a sintered body tends toshrink. Further, when the size of the ceramic particles is too large, aperformance to bond the basic materials cannot efficiently be exhibited.In a thickness direction of the porous protective layer, the size of theceramic particles may be the same or may vary.

The plate-shaped ceramic particles may be present in an area from afront surface to a back surface of the porous protective layer (over anentirety thereof in the thickness direction) or may be unevenly presentin the thickness direction. As a material of the plate-shaped ceramicparticles, minerals and clays such as talc (Mg₃Si₄O₁₀(OH)₂), mica andkaolin, glass, etc., may be used in addition to the metal oxides used asthe material of the ceramic particles as aforementioned. Theplate-shaped ceramic particles may be in a rectangular plate shape or aneedle shape, and their longitudinal size may be 5 μm or more and 50 μmor less. When the longitudinal size of the plate-shaped ceramicparticles (hereinbelow may simply be termed “length of the plate-shapedceramic particles”) is 5 μm or more, excessive sintering of the ceramicparticles can be prevented. Further, when the length of the plate-shapedceramic particles is 50 μm or less, a heat transfer path in the porousprotective layer is obstructed by the plate-shaped ceramic particles,and the sensor element can efficiently be insulated from heat.

In the thickness direction of the porous protective layer, theplate-shaped ceramic particles may be present on a back surface side ofthe porous protective layer relative to a midpoint in the thicknessdirection. By obstructing the heat transfer path near the sensorelement, heat insulation of the sensor element can be ensured. Asaforementioned, the plate-shaped ceramic particles may be present in anarea from the front surface to the back surface of the porous protectivelayer, or may be unevenly present in the thickness direction. That is,the area in which the plate-shaped ceramic particles are present may bechanged according to a purpose thereof. For example, the plate-shapedceramic particles may be present only on the back surface side relativeto the midpoint in the thickness direction, or may be present only on afront surface side relative to the midpoint in the thickness direction.Alternatively, the plate-shaped ceramic particles may be present on thefront surface side relative to the midpoint in the thickness directionin addition to the back surface side relative to the midpoint in thethickness direction. Further, an aspect ratio (longitudinal size/size ina direction orthogonal to a longitudinal direction) of the plate-shapedceramic particles may be in a range of 5 to 100. When the aspect ratiois 5 or more, the sintering of the ceramic particles can efficiently bemitigated. When it is 100 or less, decrease in strength of theplate-shaped ceramic particles themselves is mitigated, and an effect ofstrengthening the porous protective layer can be achieved.

In the thickness direction of the porous protective layer, the ceramicfibers may be present on the front surface and on the back surface sideof the porous protective layer relative to the midpoint in the thicknessdirection. In other words, the ceramic fibers may be present on a partof or over the entirety of the front surface of the porous protectivelayer and the back surface side of the porous protective layer relativeto the midpoint. For example, they may be present in the area from thefront surface to the back surface of the porous protective layer. Thatis, the ceramic fibers may be present over the entirety of the porousprotective layer in the thickness direction. Specifically, when theporous protective layer is equally divided into five in the thicknessdirection, the ceramic fibers may be present in all of the layers.However, a ceramic fiber content may be different along the thicknessdirection of the porous protective layer. Characteristics of the porousprotective layer (such as strength and heat transfer property) can bevaried along the thickness direction. As an example, the ceramic fibercontent of the porous protective layer on the front surface side may behigher than the ceramic fiber content of the porous protective layer onthe back surface side. In this case, the strength on the front surfaceside where damage caused by the gas to be detected is likely larger canbe increased, and the heat transfer path can be reduced on the backsurface side where heat insulation performance is required.

As a material of the ceramic fibers, glass may be used in addition tothe metal oxide used as the material of the plate-shaped ceramicparticles as aforementioned. A length of the ceramic fibers may be 50 μmor more and 200 μm or less. Further, a diameter (average diameter) ofthe ceramic fibers may be in a range of 1 to 20 μm. A type of theceramic fibers (such as material and size thereof) to be used may bevaried along the thickness direction of the porous ceramic layer.

The porous protective layer may comprise multiple layers along thethickness direction. That is, the porous protective layer may comprise amulti-layer structure in which the multiple layers are layered. Thenumber of the layers constituting the porous protective layer may be“2”, or may be “3” or more. For example, among the multiple layersconstituting the porous protective layer, when a layer located backmoston the back surface side (sensor element side) is considered as being afirst layer and a layer located frontmost on the front surface side (gasto be detected environment side) is considered as being a second layer,the first layer and the second layer may be in contact with each other,or other layer(s) may be interposed between the first layer and thesecond layer. As aforementioned, the porous protective layer may havevaried structures along the thickness direction depending on requiredcharacteristics. By configuring the porous protective layer in themulti-layer structure, the structure thereof may easily be varied on theback surface side (first layer) and on the front surface side (secondlayer). However, as the number of the layers in the porous protectivelayer increases, the manufacturing process becomes more complicated. Inorder to control the characteristics along the thickness direction aswell as to avoid the manufacturing process from becoming complicated,the number of the layers in the porous protective layer may be “2” or“3”. When the porous protective layer comprises the multi-layerstructure, the ceramic fibers may be included in at least one of thelayer on the front surface side (second layer) and the layer(s) on theback surface side relative to the midpoint in the thickness direction.Alternatively, the ceramic fibers may be included in all of the layers.

When the porous protective layer comprises the multi-layer structure, aporosity of the first layer may be higher than a porosity of the secondlayer. In this case, porosity adjustment may be carried out by adjustinga ratio of materials constituting each layer, or by adding apore-forming agent in raw materials for fabricating the respectivelayers. Specifically, an amount of the pore-forming agent to be added tothe raw material of the first layer may be larger than an amount of thepore-forming agent added to the raw material of the second layer. Byconfiguring the porosity of the first layer to be high, heat insulationperformance for the sensor element can be increased.

The porosity of the first layer may be 20% or more and 85% or less. Whenthe porosity of the first layer is 20% or more, sufficient heatinsulation performance can be ensured, and further, obstruction of flowof the gas to be detected into the sensor element can be prevented.Further, when the porosity of the first layer is 85% or less, sufficientstrength can be provided.

The porosity of the second layer may be 5% or more and 50% or less. Whenthe porosity of the second layer is 5% or more, obstruction of the flowof the gas to be detected into the sensor element can be prevented, andwhen the porosity is 50% or less, sufficient strength can be provided.As aforementioned, when a damage such as a crack is generated in thesecond layer, harmful component (such as metal and moisture) could reachthe first layer from this damaged portion, and the harmful componentcould flow through the first layer and damage the sensor element. Due tothis, the porosity of the second layer may be lower than the porosity ofthe first layer.

As aforementioned, the area within the porous protective layer in whichthe plate-shaped ceramic particles are present may be differentaccording to purpose. Due to this, when the porous protective layercomprises the multi-layer structure, the plate-shaped ceramic particlesmay be included in both the first and second layers, or the plate-shapedceramic particles may be included in the first layer while theplate-shaped ceramic particles are not included in the second layer.Specifically, the first layer may substantially be constituted of theplate-shaped ceramic particles and the ceramic particles (and theceramic fibers if necessary), and the second layer may substantially beconstituted of the ceramic fibers and the ceramic particles. Asaforementioned, high heat insulation performance is required in thefirst layer, and high strength is required in the second layer. Due tothis, by adding to the first layer the plate-shaped ceramic particleswith excellent balance in reinforcement of the protective layer and heatinsulation and adding to the second layer the ceramic fibers withexcellence in reinforcement without adding the plate-shaped ceramicparticles thereto, the porous protective layer as a whole may exhibitbalanced characteristics. As above, by configuring the porous protectivelayer to comprise the multi-layer structure, the characteristics in thethickness direction can easily be varied.

When the porous protective layer includes the multi-layer structure, theceramic fibers may be included in the first layer. In this case, avolume fraction of the ceramic fibers in the first layer may range from5 vol % to 25 vol % relative to a total volume of the ceramic particles,the plate-shaped ceramic particles, and the ceramic fibers. When the 5vol % or more ceramic fibers are included in the first layer, shrinkageof the ceramic particles in the first layer during firing cansufficiently be mitigated, as a result of which the porous protectivelayer can be prevented from cracked, and decrease in the strength of theporous protective layer can be prevented. Further, by configuring thevolume fraction of the ceramic fibers to be 25 vol % or less, a heattransfer path in the first layer can be obstructed, and heat insulationeffect can sufficiently be achieved. The volume fraction of the ceramicfibers is preferably 25 vol % or less. Further, as aforementioned, theceramic fibers may be present in an area from the front surface to theback surface of the porous protective layer. That is, the ceramic fibersmay be included in both the first and second layers.

As aforementioned, the porous protective layer may be constituted of theceramic particles, the plate-shaped ceramic particles, and the ceramicfibers. Other than these materials, the porous protective layer may befabricated by further using a raw material in which a binder, apore-forming agent, and a solvent are mixed. As the binder, an inorganicbinder may be used. Alumina sol, silica sol, titania sol, and zirconiasol may exemplify the inorganic binder. These inorganic binders canimprove the post-firing strength of the porous protective layer. Apolymer-based pore-forming agent, and carbon-based powder may be used asthe pore-forming agent. Specifically, acrylic resin, melamine resin,polyethylene particles, polystyrene particles, carbon black powder, andgraphite powder may be used. The pore-forming agent may be in variousforms according to a purpose, and may for example be spherical,plate-shaped, and fibrous. The porosity and pore size of the porousprotective layer can be adjusted by selecting an adding amount, size,and shape of the pore-forming agent. The solvent may be of any kind solong as it is capable of adjusting viscosity of the raw material withoutaffecting other properties of the raw material, and for example, water,ethanol, and isopropyl alcohol (IPA) may be used.

In the gas sensor disclosed herein, the aforementioned raw material maybe applied onto the surface of the sensor element, and the porousprotective layer may be disposed on the surface of the sensor element bydrying and firing the same. Dip coating, spin coating, spray coating,slit-die coating, thermal spraying, aerosol deposition (AD) method,printing, and mold casting may be used as an application method of theraw material.

Among the aforementioned application methods, dip coating isadvantageous in that the raw material can be applied evenly over anentirety of the surface of the sensor element with a single application.In dip coating, slurry viscosity of the raw material, a drawing speed ofan object to be applied (the sensor element), a drying condition of theraw material, and a firing condition are adjusted in accordance with atype of the raw material and its application thickness. As an example,the slurry viscosity is adjusted to be from 500 to 7000 mPa·s. Thedrawing speed is adjusted to be from 0.1 to 10 mm/s. The dryingcondition is adjusted such that a drying temperature is from a roomtemperature to 300° C. and drying time is 1 minute or more. The firingcondition is adjusted such that a firing temperature is from 800 to1200° C., firing time is from 1 to 10 hours, and firing atmosphere isopen air. When the porous protective layer is to be given themulti-layer structure, firing may be carried out after having formed themulti-layer structure by repeating dipping and drying, or themulti-layer structure may be formed by performing dipping, drying, andfiring for each layer.

A gas sensor 100 will be described with reference to FIGS. 1 and 2. Asshown in FIG. 1, the gas sensor 100 includes a stick-shaped sensorelement 50 extending in an X-axis direction, and a porous protectivelayer 30. The gas sensor 100 may for example be attached to an exhaustpipe of a vehicle having an engine and be configured to measure aconcentration of gas to be detected (NOx, oxygen) within exhaust gas.FIG. 1 shows one end of the gas sensor 100 (sensor element 50) in alongitudinal direction (X-axis direction).

The sensor element 50 is a limiting current type gas sensor element. Thesensor element 50 is configured of a base 80 primarily constituted ofzirconia, electrodes 62, 68, 72, 76 disposed inside and outside the base80, and a heater 84 embedded in the base 80. Front and back surfaces (Z+end surface and Z− end surface) of the base 80 each have a coating layer82 constituted of alumina disposed thereon. The coating layers 82protect the front and back surfaces of the base 80 and an outer pumpelectrode 76 to be described later.

The base 80 has oxygen ion conductivity. A space having an opening 52 isdefined within the base 80, and this is sectioned into a plurality ofspaces 56, 60, 66, and 74 by diffusion controlling bodies 54, 58, 64,and 70. The diffusion controlling bodies 54, 58, 64, and 70 are parts ofthe base 80, and are pillar-shaped members extending from both sidesurfaces (both ends in a Y-axis direction). The diffusion controllingbodies 54, 58, 64, and 70 do not completely separate the respectivespaces 56, 60, 66, and 74. The respective spaces 56, 60, 66, and 74communicate via small gaps. The diffusion controlling bodies 54, 58, 64,and 70 are configured to restrict moving speed of the gas to be detectedintroduced from the opening 52.

The space inside the base 80 is sectioned into a buffer space 56, afirst space 60, a second space 66, and a third space 74 in this orderfrom an opening 52 side. A cylindrical inner pump electrode 62 isdisposed in the first space 60. A cylindrical auxiliary pump electrode68 is disposed in the second space 66. A measurement electrode 72 isdisposed in the third space 74. The inner pump electrode 62 and theauxiliary pump electrode 68 are constituted of material(s) having a lowperformance in NO_(x) reduction. On the other hand, the measurementelectrode 72 is constituted of a material having a high performance inNO_(x) reduction (functioning as a catalyst for NO_(x) reduction).Further, the outer pump electrode 76 is disposed on a front surface ofthe base 80. The outer pump electrode 76 faces a part of the inner pumpelectrode 62 and a part of the auxiliary pump electrode 68 across thebase 80.

An oxygen concentration of the gas to be detected in the first space 60is adjusted by applying a voltage between the outer pump electrode 76and the inner pump electrode 62. Similarly, the oxygen concentration ofthe gas to be detected in the second space 66 is adjusted by applying avoltage between the outer pump electrode 76 and the inner pump electrode68. The gas to be detected of which oxygen concentration has been highlyprecisely adjusted is introduced into the third space 74. In the thirdspace 74, NO_(x) in the gas to be detected is decomposed by themeasurement electrode (NO_(x) reduction catalyst) 72, and oxygen isthereby generated. A voltage is applied to the outer pump electrode 76and the measurement electrode 72 such that an oxygen partial pressure inthe third space 74 is uniformized, and a NO_(x) concentration in the gasto be detected is detected by detecting a current value during thisvoltage application. The buffer space 56 is a space for bufferingchanges in a concentration of the gas to be detected introduced from theopening 52. In detecting the NO_(x) concentration in the gas to bedetected, the base 80 is heated to 500° C. or more by using the heater84.

The heater 84 is embedded in the base 80 so as to face positions wherethe electrodes 62, 68, 72, 76 are disposed in order to increase theoxygen ion conductivity of the base 80. The heater 84 is covered by aninsulator (not shown), and is not in direct contact with the base 80. Byincreasing a temperature of the base 80 by using the heater 84, the base(oxygen ion conductivity solid electrolyte) 80 is activated.

The porous protective layer 30 is disposed on an outer surface of thebase 80 so as to surround the spaces 56, 60, 66, and 74 in the base 80,that is, a NO_(x) concentration detection unit of the sensor element 50.The porous protective layer 30 has a two-layer structure, and includes afirst layer 10 located on a sensor element 50 side (back surface side)and a second layer 20 located on an external space side (front surfaceside) of the gas sensor 100. The first layer 10 and the second layer 20differ in their compositions.

FIG. 2 schematically shows materials that constitute the porousprotective layer 30 (first layer 10 and second layer 20). As shown inFIG. 2, the first layer 10 is disposed on front surfaces of the coatinglayers 82, and the second layer 20 is disposed on a front surface of thefirst layer 10. The first layer 10 is configured of a matrix 18, ceramicfibers 16, and plate-shaped ceramic particles 14. The matrix 18 is asintered body of ceramic particles, and bonds the ceramic fibers 16 andthe plate-shaped ceramic particles 14 being basic materials. The ceramicfibers 16 and the plate-shaped ceramic particles 14 are present by beingsubstantially evenly dispersed within the first layer 10.

Pores 12 are arranged inside the first layer 10. The pores 12 are voidsgenerated by dissipation of a pore-forming agent that was added to a rawmaterial upon fabricating the first layer 10. That is, the pores 12 aregenerated by the pore-forming agent dissipating in a manufacturingprocess (firing) of the porous protective layer 30. Aside from the pores12, voids are present in the matrix 18 in the first layer 10 as well.Porosity of the first layer 10 is adjusted to be in a range of 20 to85%.

The second layer 20 is configured of a matrix 28 and ceramic fibers 26.That is, the plate-shaped ceramic particles 14 are not present in thesecond layer 20. The matrix 28 is a sintered body of ceramic particles,and bonds the ceramic fibers 26 being basic materials. The ceramicfibers 26 are present by being substantially evenly dispersed within thesecond layer 20. A volume of the basic materials and the matrix in thesecond layer 20 is adjusted to be substantially equal to the basicmaterials and the matrix in the first layer 10. Due to this, a contentof the ceramic fibers 26 in the second layer 20 (ratio thereof to thesecond layer) is higher than a content of the ceramic fibers 16 in thefirst layer 10 (ratio thereof to the first layer).

As aforementioned, the gas sensor 100 is configured to detect the NO_(x)concentration in the gas to be detected in a state of having the base 80heated by the heater 84 embedded in the base 80. Due to this, whenmoisture in the gas to be detected makes contact with the sensor element50, the sensor element 50 (base 80) could be damaged by thermal impact,and NO_(x) detection accuracy of the sensor element 50 could bedecreased, or NO_(x) detection could be disabled. By protecting thesensor element 50 (NO_(x) detection unit) with the porous protectivelayer 30, the moisture in the gas to be detected is prevented frommaking contact with the sensor element 50, and durability of the gassensor 100 is thereby improved.

Examples

Five types of gas sensors 100 (samples 1 to 5) having the porousprotective layers 30 configured from different materials werefabricated, and characteristics of the gas sensors 100 (porousprotective layers 30) were evaluated. FIG. 3 shows formulations ofslurries used to fabricate the respective samples, and FIG. 4 showsevaluation results of the respective samples.

Application of the porous protective layer 30 onto each of the sensorelements 50 was respectively carried out by dipping. Specifically, aslurry for the first layer (first slurry) and a slurry for the secondlayer (second slurry) were prepared, one end of the sensor element 50was immersed in the first slurry and a 300 μm first layer was therebyformed. After this, the sensor element 50 was placed in a dryer, and thefirst layer was dried for one hour at 200° C. (in open air atmosphere).Next, a portion of the sensor element 50 where the first layer wasformed was immersed in the second slurry and a 300 μm second layer wasthereby formed. After this, the sensor element 50 was placed in thedryer, and the second layer was dried for one hour at 200° C. (in openair atmosphere). Then, the sensor element 50 was placed in an electricfurnace, and the first and second layers were fired for three hours at1100° C. (in open air atmosphere).

The first slurry will be described. The first slurry was prepared bymixing alumina fibers (average fiber length: 140 μm), plate-shapedalumina particles (average particle diameter: 6 μm), titania particles(average particle diameter: 0.25 μm), alumina sol (alumina content:1.1%), acrylic resin (average particle diameter: 8 μm), and ethanol.

The alumina fibers and the plate-shaped alumina particles correspond tothe basic materials, and the titania particles correspond to a bondingagent. In the respective samples (samples 1 to 5), the alumina fibers,the plate-shaped alumina particles, and the titania particles weremeasured to be in volume ratios shown in FIG. 3. The alumina solcorresponds to a binder (inorganic binder). The alumina sol was added by10 wt % relative to a total weight of the basic materials and thebonding agent. The acrylic resin corresponds to the pore-forming agent,and was added such that the pore-forming agent was 60 vol % relative toa total volume of the basic materials, the bonding agent, and thepore-forming agent. The ethanol is a solvent, and was adjusted such thatviscosity of the first slurry was 2000 mPa·s.

The second slurry will be described. The second slurry was prepared bymixing alumina fibers, titania particles, alumina sol, acrylic resin,and ethanol. The alumina fibers, the titania particles, the alumina sol,the acrylic resin, and the ethanol used hereof were the same as thoseused in the first slurry. In the second slurry, the alumina fiberscorrespond to the basic materials and the titania particles correspondto the bonding agent. In the respective samples (samples 1 to 5), thealumina fibers and the titania particles were measured to be in volumeratios shown in FIG. 3. The alumina sol (inorganic binder) was added by10 wt % relative to a total weight of the basic materials and thebonding agent. The acrylic resin (pore-forming agent) was added suchthat the pore-forming agent was 20 vol % relative to a total volume ofthe basic materials, the bonding agent, and the pore-forming agent. Theethanol (solvent) was adjusted such that viscosity of the second slurrywas 2000 mPa·s.

The alumina fibers and the plate-shaped alumina particles correspond tothe basic materials and the titania particles correspond to the bondingagent. In the respective samples (samples 1 to 5), the alumina fibers,the plate-shaped alumina particles, and the titania particles weremeasured to be in volume ratios shown in FIG. 3. The alumina solcorresponds to the binder (inorganic binder). The alumina sol was addedby 10 wt % relative to a total weight of the basic materials and thebonding agent. The acrylic resin corresponds to the pore-forming agent,and was added such that the pore-forming agent was 60 vol % relative toa total volume of the basic materials, the bonding agent, and thepore-forming agent. The ethanol is a solvent, and was adjusted such thatviscosity of the first slurry was 2000 mPa·s. In the sample 5, aluminaparticles (average particle diameter: 20 μm) were used as the basicmaterial.

Porosity measurement, post-firing appearance observation, and wettingtest were carried out on the samples 1 to 5. The evaluation results areshown in FIG. 4. The porosity measurement was carried out for each ofthe first and second layers. Specifically, each layer was observed usinga Scanning Electron Microscope (SEM), an observed image was binarizedinto voids and portions other than the voids, and a ratio of the voidsrelative to an entirety of the image was calculated. The post-firingappearance observation was carried out by firing the respective samplesand cooling them to a room temperature, and thereafter a crackgeneration in the porous protective layer (second layer) was visiblychecked. In FIG. 4, “◯” is given to the sample that was free of crack,and “×” is given to the sample with crack.

In the wetting test, each gas sensor 100 was driven in open air and 20μL of water droplets were dripped onto its porous protective layer 30,and morphological changes in the porous protective layer 30 and thesensor element 50 were checked. Specifically, the current value flowingbetween the outer pump electrode 76 and the inner pump electrode 62 wasmeasured in the state where the heater 84 was electrically conducted tobring inside of the first space 60 to a heated state, and the voltagewas applied between the outer pump electrode 76 and the inner pumpelectrode 62 such that the oxygen concentration in the first space 60 isuniformized. 20 μL of water droplets were dripped onto the front surfaceof the porous protective layer 30 after the current value stabilized.After this, electric conduction of the heater 84 was stopped, and themorphological changes in the porous protective layer 30 and the sensorelement 50 were checked.

The morphological change in the porous protective layer 30 was checkedby visually observing presence/absence of a crack, delamination, and thelike. Further, as the morphological change in the sensor element 50,presence/absence of crack generation was checked by X-ray CT. In FIG. 4,“◯” is given to the sample that was free of crack and delamination, and“×” is given to the sample with crack or delamination. A changing amountin the current value flowing between the outer pump electrode 76 and theinner pump electrode 62 while the water droplets were dripped onto theporous protective layer 30 was also checked.

As shown in FIG. 4, post-firing crack generation in the porousprotective layer 30 (second layer 20) was confirmed in the sample inwhich the first layer 10 does not include the alumina fibers (sample 4)and in the sample in which the second layer 20 does not include thealumina fibers (sample 5). Contrary to this, post-firing crackgeneration was not confirmed in the samples 1 to 3. This resultindicates that a firing shrinkage amount was equalized by containing thealumina fibers (ceramic fibers) over an entirety of the porousprotective layer 30 in a thickness direction, and the crack generationwas thereby prevented. That is, it is expected as that since the firstlayer 10 or the second layer 20 of the samples 4 and 5 does not includethe alumina fibers, a firing shrinkage amount of a layer not includingthe alumina fibers became large, resulting in a difference in firingshrinkage amounts in the thickness direction, and a crack was therebygenerated.

Further, crack generation in the sensor elements 50 after the wettingtest was confirmed in the samples 4 and 5. Contrary to this, no crackwas confirmed after the wetting test in the sensor elements 50 of thesamples 1 to 3. It is expected as that in each of the samples 4 and 5,moisture had reached the sensor element 50 through the crack generatedin the porous protective layer 30 after the firing, and crack wasgenerated in the sensor element 50 by thermal impact. The sample 5exhibited crack and delamination in the second layer 20 during thewetting test, and the first layer 10 was exposed. On the other hand,progression of the crack in the second layer 20 of the sample 4 ascompared to its pre-wetting test state was not confirmed. The samples 1to 4 each have the alumina fibers (ceramic fibers) included in theporous protective layer 30 on a front surface side (second layer 20).The results of FIG. 4 indicate that strength on the front surface sidewas increased by the alumina fibers, as a result of which no crack wasgenerated (progression of the crack did not take place) in their porousprotective layers 30.

As shown in FIG. 4, the samples 1 to 3 each achieved excellent resultsin the post-firing appearance observation and the wetting test. Ascompared to the sample 3, the samples 1 and 2 had smaller changingamounts in the current value that flowed between the pump electrodes 76,62 during the wetting test. As shown in FIG. 3, in each of the samples 1and 2, the alumina fiber content in the first layer 10 is less than thatof the sample 3. Specifically, in each of the samples 1 and 2, a volumeof the alumina fibers is 25 vol % or less relative to the total volumeof the basic materials and the bonding agent (alumina fibers,plate-shaped alumina particles, and titania particles). This resultindicates that heat insulating performance of the porous protectivelayer 30 is further increased by adjusting the volume of the aluminafibers to be 25 vol % or less relative to the total volume of the basicmaterials and the bonding agent.

Specific examples of the present disclosure have been described indetail, however, these are mere exemplary indications and thus do notlimit the scope of the claims. The art described in the claims includemodifications and variations of the specific examples presented above.Technical features described in the description and the drawings maytechnically be useful alone or in various combinations, and are notlimited to the combinations as originally claimed. Further, the artdescribed in the description and the drawings may concurrently achieve aplurality of aims, and technical significance thereof resides inachieving any one of such aims.

1. A gas sensor comprising: a porous protective layer disposed on asurface of a sensor element, wherein the porous protective layerincludes ceramic particles and ceramic fibers, and in a thicknessdirection of the porous protective layer, the ceramic fibers are presenton a front surface of the porous protective layer and on a back surfaceside of the porous protective layer relative to a midpoint in thethickness direction.
 2. The gas sensor according to claim 1, wherein theporous protective layer includes plate-shaped ceramic particles.
 3. Thegas sensor according to claim 2, wherein in the thickness direction ofthe porous protective layer, the plate-shaped ceramic particles arepresent on the back surface side relative to the midpoint in thethickness direction.
 4. The gas sensor according to claim 3, wherein aceramic fiber content is higher on a front surface side of the porousprotective layer than on the back surface side of the porous protectivelayer.
 5. The gas sensor according to claim 4, wherein the porousprotective layer comprises a multi-layer structure including a firstlayer located on the back surface side and a second layer located on afront surface side, and a porosity of the first layer is higher than aporosity of the second layer.
 6. The gas sensor according to claim 5,wherein the first layer includes the ceramic fibers, and a volumefraction of the ceramic fibers in the first layer ranges from 5 vol % to25 vol % relative to a total volume of the ceramic particles, theplate-shaped ceramic particles, and the ceramic fibers.
 7. The gassensor according to claim 6, wherein the first layer and the secondlayer are in contact with each other.
 8. The gas sensor according toclaim 1, wherein a ceramic fiber content is higher on a front surfaceside of the porous protective layer than on the back surface side of theporous protective layer.
 9. The gas sensor according to claim 1, whereinthe porous protective layer comprises a multi-layer structure includinga first layer located on the back surface side and a second layerlocated on a front surface side, and a porosity of the first layer ishigher than a porosity of the second layer.
 10. The gas sensor accordingto claim 9, wherein the first layer includes the ceramic fibers, and avolume fraction of the ceramic fibers in the first layer ranges from 5vol % to 25 vol % relative to a total volume of the ceramic particles,the plate-shaped ceramic particles, and the ceramic fibers.
 11. The gassensor according to claim 5, wherein the first layer and the secondlayer are in contact with each other.